The invention relates to a method of forming metallic components by forging, rolling, drawing, bending, extruding, gas forming, stamping or other method involving deformation of metal into a desired metallic shape.
In forming intricate shapes and thin shapes from metal by traditional methods such as stamping and extruding, ductility of the metal and has been a limiting factor in preventing the manufacture of small shapes, thin shapes, and intricate shapes of unitary structure (i.e., without metals joining). In particular, when metal of a particular thickness is deformed beyond a certain point, it ruptures.
Superplasticity, generally, is the capacity of metal to undergo larger uniform plastic deformation without rupture. Structural superplasticity has been defined many ways, but for purposes of this description refers to the ability of a metal to undergo more than about 200% elongation under tension without rupturing. High strain rate superplasticity is defined as a minimum strain rate of 10xe2x88x922 sxe2x88x921 (Glossary of Terms Used in Metallic Superplastic Materials, JIS-H-7007, p. 3, Japanese Standards Association, Tokyo, Japan (1995)).
Metals have been treated by a variety of methods to impart superplasticity to enable the formation of small and intricate components. As a general proposition, with aluminum alloys, superplasticity is imparted by treating the metal to yield a fine grain size of less than about 20 microns and high angle grain boundaries.
The widespread use of superplastic forming of aluminum alloys is hampered by the slow optimum strain rate for superplasticity, particularly in commercial aluminum alloys. In particular, optimum strain rates typically are 2 to 10xc3x9710xe2x88x924 sxe2x88x921 for conventionally fabricated superplastic aluminum alloys. Using this material at higher strain rates, ductility is reduced and deformation is more of a diffuse necking rather than uniform extension by superplastic forming. In addition, elaborate thermo-mechanical processing has been necessary to obtain a microstructure conducive to superplastic deformation. This processing limits the versatility for subsequent superplastic forming and adds considerably to the cost.
With conventional superplastic materials, a through thickness fine grain microstructure can only be obtained in relatively thin sheet, approximately 2.5 mm or less. Further, for structural aluminum alloys, the practical limit for grain size has been on the order of 8 microns. With the friction stir processing of the invention, the grain size achievable is significantly less, on the order of down to about 3 microns or below, resulting in higher uniform elongation, facilitating fabrication of more complex shapes and at higher strain rates.
Paton et al. (J. Metals, 34(8), 21 (1982)) developed a four-step thermo-mechanical processing treatment to obtain grain sizes in the range of 8-14 microns in commercial 7075 and 7475 aluminum alloys. The 7075 Al alloy exhibited optimum superplastic behavior at a temperature of 516 C. and a strain rate of 2xc3x9710xe2x88x924 sxe2x88x921.
Xinggang et al. (Metall. Trans. 24A, 2596 (1996); Acta Metall. Mater. 41, 2721 (1993)) have refined the thermo-mechanical processing of 7075 Al alloy to increase the optimum strain rate to 8.3xc3x9710xe2x88x924 sxe2x88x921 at 510 C. The improved thermo-mechanical processing involved solution treatment, averaging, multiple warm rolling passes (200-220 C.) with intermittent re-heating and a final recrystallization treatment. The thermo-mechanical processing is complex and still the optimum superplastic strain rate is an order of magnitude slower than desirable for widespread use of superplastic forging/forming of components in automotive and other industries.
Severe plastic deformation (SePD) processing approaches such as equal channel angular extrusion (ECAE) have been used to achieve superplasticity. Berbon et al. have used ECAE to obtain high strain rate superplasticity in a commercial 1420 Al alloy at significantly lower temperature. (Metall. Mater. Trans. 29A, 2237 (1998)). A typical grain refinement schedule by ECAE consists of 8-10 passes at intermediate temperatures. An even higher shift in optimum superplastic strain rate and decrease to lower temperature were demonstrated by Mishra et al. in a 1420 Al alloy processed by torsional strain (TS)-SePD. (J. Metals 51(1), 37 (1999)). TS-SePD produces a nanocrystalline (average grain size less than 100 nanometers) microstructure but the process is limited to a very small specimen size, typically 20 mm in diameter and 0.5-1 mm thick, and is not practical for commercial superplastic forming operations.
Accordingly, there is a practical need to develop processing techniques to shift the optimum superplastic strain rate to at least 10xe2x88x922 sxe2x88x921 in commercial aluminum alloys and other metals produced by casting and powder metallurgical techniques.
It is an object of the invention, therefore, to provide a process for enhancing the superplasticity of metal and forming shapes therefrom; which process is suitable for use with aluminum alloys; which process is economical; which process is suitable for use with thin cross sections; which process is suitable for use with thick cross sections; which process achieves high strain rate superplastic forming of commercial alloys; which process achieves selective superplastic forming; which process is suitable for use with cast sheets; which process is suitable for use with hot-pressed powder metallurgy sheets; and which process imparts superplasticity in contoured sheets to achieve uniform thickness.
Briefly, therefore, the invention is directed to a method for producing a shaped metallic component involving friction stirring at least a segment of a single piece of bulk metal to impart superplasticity thereto and thereby yield a single superplastic metal blank from said single piece of bulk metal; and deforming the superplastic metal blank to yield a shaped metallic component.
Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.